http://lostcreekllamas.com/faq.htm
Llamas are not as hefty as they appear and none are usable for pleasure riding by adults. An exceptionally well-built and well-conditioned pack llama can be trained to carry a lightweight adult in case of emergency.
A well-trained, responsive gelding led by an adult makes a highly satisfactory trail mount for a child. Because of llamas’ intelligence and highly flexible, strong necks, they can easily avoid responding to directions and so allowing small children to ride a llama controlled by reins alone is unwise under most circumstances.
Although some people advocate packing llamas at two, and others as young as six months, even the hard-working South American llamas are not packed before three-and-a-half. There is ample evidence that bones are still growing and maturing after three years, and so it is best to begin with very light loads (20-35 lbs) after three years and increase the load to around 20% of body weight only after the llama’s fourth birthday.